Composite fibers and microfibers made therefrom as well as different processes for their manufacture are well known in the art.
The composite fibers are manufactured in general by combining at least two incompatible fiber-forming polymers via extrusion followed by optionally dissolving one of the polymers from the resultant fiber to form microfibers.
U.S. Pat. No. 3,700,545 discloses a multi-segmented polyester or polyamide fiber having at least 10 fine segments with cross sectional shapes and areas irregular and uneven to each other.
The spun fibers are treated with an alkali or an acid to decompose and at least a part of the polyester or polyamide is removed.
Described is a complex spinnerette for the manufacture of such fibers.
U.S. Pat. No. 3,382,305 discloses a process for the formation of microfibers having an average diameter of 0.01 to 3 micron by blending two incompatible polymers and extruding the resultant mixture into filaments and further dissolving one of the polymers from the filament. The disadvantage if this process is that the cross section of these filaments is very irregular and uneven and the islands, which form the microfibers after the hydrolysis, are discontinuous, which means that they are not continuous over the length of the composite fibers.
U.S. Pat. No. 5,120,598 describes ultra-fine polymeric fibers for cleaning up oil spills. The fibers were produced by mixing an polyolefin with poly (vinyl alcohol) and extruding the mixture through a die followed by further orientation. The poly (vinyl alcohol) is extracted with water to yield ultra-fine polymeric fibers. A disadvantage of this process is the limitation of the polymers to the polyolefin family because of their relative low melting point. At higher temperatures which are necessary for the extrusion of polyamides or polyesters, the poly (vinyl alcohol) decomposes.
EP-A-0,498,672 discloses microfiber generating fibers of island-in-the-sea type obtained by melt extrusion of a mixture of two polymers, whereby the sea polymer is soluble in a solvent and releases the insoluble island fiber of a fineness of 0.01 denier or less. Described is polyvinyl alcohol as the sea polymer, which limits the application to the polyolefin polymer family because of their relative low melting point. Another disadvantage is that by the process of melt mixing the islands-in-the-sea cross section is irregular and uneven and the islands, which form the microfibers after the hydrolysis, are discontinuous, which means that they are not continuous over the length of the composite fibers.
U.S. Pat. No. 4,233,355 discloses a separable unitary composite fiber comprised of a polyester or polyamide which is insoluble in a given solvent and a copolyester of ethylene terephthalate units and ethylene 5-sodium sulfoisophthalate units, which is soluble in a given solvent. The composite fiber was treated with an aqueous alkaline solution to dissolve out at least part of the soluble polymer component to yield fine fibers. The cross sectional views of the composite fibers show an "islands-in-a-sea" type, where the "Islands" are the fine fibers of the insoluble polymer surrounded by the "sea" of the soluble polymer. The highest described number of segments or "islands" are 14 and the lowest described fineness were 108 filaments having a total fineness of 70 denier which corresponds to 0.65 denier per filament.
Object of the present invention is to provide a composite fiber with a cross-section having at least 19 segments of a water-insoluble polymer, surrounded by a water dissipatable polymer, which is not limited to polyolefins as the water-insoluble polymer and which is applicable to polymers with a higher melting and processing temperature and wherein the segments of water insoluble polymer are uniformly distributed across the cross-section of the composite fiber and are continuous over the length of the composite fiber.
Another object was to provide a process for the manufacture of such a composite fiber.
Another object was to provide a process for the manufacture of microfibers of a fineness of not greater than 0.3 denier from the composite fibers.